1. Field of the Invention
The present invention relates to a technique for processing a line drawing, and in particular to a technique for applying color to a line drawing.
2. Description of the Background Art
Japanese manga is one of typical examples of monochrome (uncolored) line drawings. Generally different from American and European comics, manga is a monochrome line drawing having Japanese distinct characteristics. In other words, manga expresses shades of gray (gradation) and characters' feeling by the use of patterns such as various kinds of dots, solid fills (painting a given range with a single color without any clearance left), and effective lines; and drawing lines such as contours. Manga is thus quite different in property from comics which are mostly presented in colors.
Conventionally, manga supplied to the market is printed in paper. Because of the high cost of color printing, manga has been created only in monochrome, except opening color pages of magazines and the like in Japan.
However, with recent development of communications technology using cellular phones or other terminal equipment, there is a rapidly increasing number of websites from which digitized manga is available over communication lines, and as a result, there is a greater number of opportunities to see manga on LCD monitors or the like. This is followed by the increasing demand for colored manga. In addition, since people in countries outside Japan are unfamiliar with monochrome manga, it is preferable to apply color to monochrome manga in order to expand the manga business internationally.
In the process of applying color to a line drawing such as manga, an operator performs a given operation while visually checking a digitized line drawing in the form of digital data displayed on a monitor, to thereby select and apply color to a specific range of an image. Such a coloring operation, however, has taken an enormous amount of time because manga or the like often contains a dotted screen tone of tiny dots applied, a number of thin drawing lines, and a number of line disconnections in drawing lines indicating contours.
In this way, for the manual coloring of a line drawing by the operator, a number of complicated process steps are required, so that there is a need for a technique for automating the coloring process. There have been so far several techniques for semi-automating the coloring process (e.g., Japanese Patent Application Laid-open No. 10-293855.)
For example, Japanese Patent Application Laid-open No. 10-293855 discloses a technique for setting a central pixel in a line drawing and applying color to a circular region of a given radius with the central pixel as the center. More specifically, the radius of the above circular region is determined based on the result of measurement of distances from the central pixel to edges (e.g., drawling lines) in a plurality of directions.
In the above technique, however, for a region with a number of thin drawing lines (e.g., a “face” region including forelock hairs drawn in detail), a number of thin drawing lines may be detected as edges. Thus, it is difficult to color gaps between those drawing lines.
Also in the above technique, only the edges in a plurality of directions with reference to the central pixel are detected, which may result in excessive coloring beyond the range intended by the operator. It is thus necessary for the operator to monitor the conditions of coloring and, if there is any problem, to stop the coloring process as needed.
In many cases, manga often contains a number of breaks (line disconnections) in drawing lines. For example, considering two (or more) regions with a line-disconnection-containing drawing line sandwiched therebetween, it is preferable in some cases to apply different colors to those regions and in other cases to apply the same color to those regions as a single region. To be more specific, when the contour drawn between the “face” and “neck” of a character in a manga includes any line disconnection, the same skin color may be applied to the face and neck regions in some cases. However, since in the above technique, line disconnections are not taken into consideration, it is difficult to apply, at a time, the same color to a plurality of regions with a line-disconnection-containing drawing line sandwiched therebetween.